How come https://riot.im (Matrix) manages to sync between devices AND have E2E, while also being federated?
That's not what you get when you have a secure system, that's what you get when you design a system that can collect (and possibly monetize) the data of millions of users.
I believe that making sync work with E2E bring either security issues or more burden on the user; I would like them in telegram, but I also like the "if you send this message you exactly the device it go to, not an old laptop i forgot in the office, just my phone". it is meant to be secure on a device level.
That's not what I meant. Yes you can have sync and E2E. With different trade-offs.
Telegram is secure from device to device, not from account to account. If I send you a secure message from my iPad I don't have to worry about the web session I opened a week ago on someone else laptop.
Signal and WhatsApp both support multi-device encrypted chats. Signal is better than WhatsApp in this respect as your primary device doesn't need to be online for it to work.
Signal multi-device support is very limited. Doesn't support multiple mobile devices. Primary device must be a phone, all others must be desktop computers with Chrome.
That's not what you get when you have a secure system, that's what you get when you design a system that can collect (and possibly monetize) the data of millions of users.